The Armageddon is the strongest battlecruiser at the imperial navy's disposal. It does NOT need a nova cannon.
When i look at the three battlecruisers, i view them as different tools for different jobs. The mars literally says "stay back and pound the enemy from afar", The Overlord says "Long range support" and the armageddon screams "GET IN AND SCREW PEOPLE UP IN THE FACE!!!"